Its a human thing.

Geeks can get fired up and support robot research, but most people dont. You need something that catches the popular imagination. A trip to find life there by people is more likely to do that than the bot.

Mind, I think the bots are cool as hell, but then I am a certified geek so I dont count.

One of the great things about NASA is it is almost the only major tech advance, not done during a war, or gearing up for one.